1. Field of the Invention
This invention relates generally to the field of single-photon detection, and more particularly to an apparatus and method for allowing the driving of avalanche photodiodes in several single-photon detection operating modes with a single electronic circuit by enabling the tunability of the rise-time of the electrical signal used for activating the photodiode in Geiger mode.
2. Description of the Prior Art
A single-photon is the elementary quantum of the electromagnetic field. Recently the detection of single-photons has generated a lot of interest. On the one hand, it allows reaching the ultimate sensitivity in the detection of light. This is useful in applications such as spectroscopy, single-molecule detection, range finding or photoluminescence where the probed system emits or reflects only a small quantity of light, which cannot be detected with a classical detector.
Single-photon detection also allows the exploitation of the quantum properties of photons to study quantum optics and to perform tasks which are not allowed by classical physics, such as quantum key distribution (Nicolas Gisin, Grégoire Ribordy, Wolfgang Tittel, and Hugo Zbinden, “Quantum Cryptography”, Rev. of Mod. Phys. 74, (2002), the content of which is incorporated herein by reference thereto).
Good single-photon detectors working in free-running mode exist for visible and near-infrared wavelengths up to about 900 nm. At these wavelengths, Silicon avalanche photodiode operated in Geiger mode (S. Cova, M. Ghioni, A. Lacaita, C. Samori and F. Zappa, “Avalanche photodiodes and quenching circuits for single-photon detection, Appl. Opt. 35 (1996), the content of which is incorporated herein by reference thereto) achieve simultaneously high detection efficiencies (up to 80%) and low dark count probabilities (below 100 counts/s). They also feature low timing jitter (50 to 500 ps depending on the type of detector). Their afterpulsing probability is low, which makes it possible to use a short deadtime after each detection and in turn to achieve counting rates as high as ten megahertz.
Unfortunately, the situation is radically different beyond a wavelength of 900 or 1000 nm. Silicon based detectors cannot be used at these wavelengths, because the energy of the photons is smaller than the bandgap of this material, making band to band excitation by optical absorption impossible.
These wavelengths are however of great scientific and technological interest. As attenuation reaches a minimum around 1550 nm in optical fibers, most practical optical communication systems, both classical and quantum, work at a wavelength close to this wavelength. For quantum communications, good single-photon detectors are needed to build receivers. In the case of classical communications, such detectors are useful for characterization instruments, such as optical time domain reflectometers. When shined into an eye, 1550 nm light is absorbed in the vitreous humor, before it reaches the retina. Because of this important property, this wavelength is considered as “eye safe”. It is used in applications such as LIDAR and range finding, where strong light pulses are sent into the atmosphere and can possibly be reflected in uncontrolled and hazardous manners. As the reflected and backscattered intensities can be low, single-photon detectors are also useful in this kind of applications.
In most cases where single-photon detection is needed at a wavelength beyond 1000 nm, people use InGaAs/InP avalanche photodiode operated in Geiger mode (Grégoire Ribordy, Jean-Daniel Gautier, Hugo Zbinden and Nicolas Gisin, “Perfomance of InGaAs/InP avalanche photodiode as gated-mode photon counters”, Appl. Opt. 37 (1998), the content of which is incorporated herein by reference thereto). These detectors unfortunately exhibit a high afterpulsing probability due to fabrication quality issues. Because of this, free running operation is quite difficult to implement. Hence, they are used in gated mode in a large majority of the applications. The gated operating mode is implemented with a gate pulse driver which activates the avalanche photodiode in Geiger mode during the duration of the gate applied by the driver. The fabrication quality of InGaAs/InP avalanche photodiodes has, nevertheless, been strongly improved those last few years. Free running operation has been demonstrated (Robert Thew, Damien Stucki, Jean-Daniel Gautier, Hugo Thinden, Alexis Rochas, “Free-running InGaAs/InP avalanche photodiode with active quenching for single photon counting at telecom wavelengths”, Appl. Phy. Lett. 91 (2007), the content of which is incorporated herein by reference thereto). Even if the free-running mode has lower noise performances than the gated mode, it is more and more appreciated by people working with asynchronous optical signals.
The best for a commercial single-photon device based on InGaAs/InP avalanche photodiodes is to give the possibility to the user to operate it either in gated mode or free-running modes. However, those two operating modes have some antagonistic requirements. Especially, when the detector is activated with short gates (width <5 ns), the rise-time of the gate needs to be very short. If the same rise-time value is used for long gate (>5 ns) or free-running operations, the overshot of the electrical signal implies an oscillation of the efficiency of the detector for a period of several nanoseconds after the rising edge of the electrical signal. This phenomenon avoids the capability of effectively driving the avalanche photodiode in all the operating modes with a single electronic circuit if it is impossible to tune the rising time of the electrical signal.
What is needed therefore, to allow the effective driving of avalanche photodiodes in all the possible single-photon detection operating modes with a single electrical driving circuit, is a technique which enables the tunability of the rising time of the electrical signal used for activating the photodiode in Geiger mode.